The elastomer of a weighing sensor mainly comprises three parts, that is, a first stress surface for transferring acting force, a second stress surface for transferring reaction force, and a strain rod connected between the first stress surface and the second stress surface. In a general weighing sensor, one or two stress surfaces have to be riveted or screw-fixed onto a rigid base frame or a supporting beam for transferring the acting force and the reaction force.
The elastomer of a self-supporting weighing sensor is shaped by punching a piece of sheet metal into two stress surfaces and a strain rod, wherein one of the stress surfaces forms a bearing frame to make the sensor itself stably disposed on the base frame of the scale body for directly transferring acting force to cut down the cost of the sensor and the whole electronic scale without fixing a supporting beam onto the base frame via a hard connection.
In order to eliminate or reduce partial load and repeatability error of the self-supporting weighing sensor, the other stress surface of the sensor, which transfers reaction force, transfers the reaction force in a manner of point contact.
For the present, there are three kinds of common self-supporting weighing sensors used for the electronic scales. The first kind is of “M” type structure, as shown in FIG. 12, of which the particular structure is described in detail in U.S. Pat. No. 5,929,391, published on Jul. 27, 1999; the second kind is shown in FIG. 13, of which details are described in CN patent application No. 96106220.7, published on Jul. 2, 2003; and the third kind is of “E” type structure, as shown in FIG. 14, of which details are described in CN patent No. 200420015320.6, published on Apr. 6, 2005.
The first kind, i.e., the sensor with “M” type structure, has a bearing frame for self-supporting and a load-supporting point for transferring the reaction force, and is convenient for installation. However, it has two strain rods, and the strain gauge must be bonded to each of them to ensure precision, which highly increases the cost. The second and the third types of sensors introduced in the above patent documents both have bearing frames for planar self-supporting, but the other stress surface can not directly be stressed, so load-supporting portions are necessarily added to achieve point stressing, which is also costly.